Voltage monitor system



July 23, 1963 G. L. cLAPPl-:R 3,098,999

VOLTAGE MONITOR SYSTEM Filed Deo. 11, 1958 United States Patent O 3,098,9 VOLEAGE B/iNiR SYSTEM Genung Leland Ciapper, Vestal, N55., assigner to Enternational Business Machines Corporation, New York, NX., a corporation of New York Fiied Dec. 1i, i953, Ser., No. 779,737 2 Qiaims. (Cl. 34e-248) This invention relates to a voltage monitor system and more particularly to a system which is capable of monitoring and comparing the voltage present at a single point, or a number of points, of each of a plurality of locations with a reference rvoltage and providing an indication of the location and particular point at which the monitored Voltage diiers from the reference voltage and a further indication of the magnitude of the difference.

It is often desirable to monitor the voltage present at different locations in a complex electronic system and to compare its magnitude With the magnitude of a reference voltage. Once this comparison has taken place, it is then desirable to provide an indication of any location Where the magnitude oi t e voltage present diiiers from the magnitude of the reference voltage. Each of the locations being monitored may have either `a single or -a plurality of voltage points to be checked. One example of an environment where such monitoring would be desirable would be in a complex digital computer Where there are a number of locations at which voltage levels of the same magnitude are present. A system which could perform this type of monitoring and checking would `also have utility in a variety of electrical systems.

The present invention provides a system for monitoring the voltages present at a plurality of locations and comparing their magnitudes -with a reference voltage. in accordance with the operation of the invention, an error voltage is generated as a result of the voltage monitored at a particular location differing from the reference voltage. The magnitude of the error signal is dependent upon the difference in potential between the two voltages. The error signal produced by the comparison is then ampliiied in a novel sense amplifier which is capable of producing t-wo distinct levels of output signal corresponding to an error signal of small magnitude called the tine error and an error signal of larger magnitude called the coarse error. Provisions are made whereby the coarse error signal, which is representative of a relatively large difference between the magnitude of the voltage at the location and the reference voltage, is used to disable or shutdown the circuits which produce the voltage for the various locations. An indicator circuit is also provided to indicate the specic location which caused the error signal, either coarse or tine. The monitoring system of the present invention also provides -a test circuit by means of which an error signal is generated When each of the locations is checked. The test circuit enables a check to be made of the indicator circuit.

It is therefore an object of this invention to provide a voltage monitoring system which is capable of monitoring the voltages at ya plurality of locations.

Another object of this invention is to provide a voltage monitoring system in which the voltage present at a plurality `of locations is sequentially compared with a reference voltage and an error signal is produced when the comparison shows a discrepancy.

3,098,999 Patented July 23, i963 it is still another object of this invention to provide a voltage monitoring system which is capable of monitoring the voltages present at a plurality of locations and producing an indication of the particular location at which the voltage monitored differs from a reference voltage.

Yet another object of the invention is o provide a voltage monitoring system which utilizes a nvel sense ampliier which is capable of producing a plurality of separate output signals in accordance with the magnitude of the error signal which is produced `as a result of the comparison of `a monitored voltage with a reference voltage.

Other objects of the invention will be pointed out in the following description and claims and illustrated in the accompanying drawing, which discloses, by way of eX- arnple, the principle of the invention and the best mode, which has been contemplated, of applying that principle.

Referring to the drawing, a plurality of locations 10a, 1Gb, No, itin Lare shown. Each of ythe locations lila, itin is representative of a point in a complex electronic system at which a voltage of substantially the same magnitude is present. it is the magnitudes of these voltages which are to be periodically checked against a reference voltage. IThe locations lila, 1011 may, Ifor example, represent the input or output circuits of a series of direct current ampliiiers, or the outputs of a plurality of bistable circuits, or :any other environment where the voltage present at a plurality of locations Vis to be tested. As illustrated, only a single point is to be monitored at each of the locations 10a, l10n. It lshould be realized that the system may be expanded to monitor a plurality of points at each of the locations lila, itin, as will later be explained.

The Voltage present at each of the locations lila, 1611 is to be compared with -a reference voltage 12 which is preferably supplied from a variable potential source (not shown). The potential source for the reference potential 12 is preferably variable in order to make the monitoring system more rileXible in the range of operation. Each of the locations 16a, i011 is connected to a respective movable contact 14a, 14h, 114e 14n, of a relay or distributor type device (not shown) by means of an integrating network formed -by the respective resistors Ma, 16h, loc, 1161i and capacitors 17a, 17b, 17C I1711. The lower ends of capacitor 17a, 1717, 17e '1711, are connected to a source of common reference potential, such as ground 19. The integrating networks formed by resistors 116 and capacitors 17 are used to provide ltering and averaging for the voltages being monitored. The time constant of the integrating network is short enough, however, so that D.C. ripple voltages present at the locations :lila y10ft will be detected.

The reference voltage l2 is supplied through a resistor 22 to a junction point 21 where the comparison of the Voltage monitored at the various locations lila 1G12 and the reference voltage 12 is accomplished. A capacitor 23 is connected between the junction point 21 and ground 19 in order to iilter the reference voltage I12.

Each of the locations 16a, 10b, ltlc 10ft is sequentially connected to a common line 26 by means of the stationary contact points 28a, 28h, 28C 2821 of the relay or distributor device. For the remainder of the speciiication the use of a distributor device is assumed, but it should be realized that any equivalent type of device may be used. The distributor device sequentially makes and breaks contact between the movable contacts 14a, 1412, 14C 14n and the stationary contact points 28a, ZSb, 23C 2811 thereby sequentially applying the voltage present at the locations a, 1Gb, 16e 1011 to the line 26. The line 26 is in turn connected to the voltage comparison junction point by means of resistor 3d and through the primary winding 32 of a pulse transformer 33. ln this manner, the voltages present at the various locations 19a 114m are sequentially applied to the junction point 21 yfor comparison with the reference voltage 12 for the time that the respective movable contact 14a 14n makes contact with the stationary contact point 23a 23a. lf the voltage monitored at any of the locations ta 1311 differs from the reference voltage 12, a voltage pulse is produced in the primary winding 32 of the pulse transformer 33. The voltage at Ia respective location may vary from its ideal value, which is approximately equal to the reference voltage 12, due to component aging or failure, variation in the location supply voltage or any of a number of other reasons. The voltage pulse which is formed has a time duration which is equal to the period of contact between the contact points 14 and 28 and a magnitude which is equal to the difference in voltage between the reference voltage 12 and the voltage at the particular location being monitored. If the voltage monitored at a respective location 10a 19m `is substantially equal to the reference voltage 12, no pulse is produced in the primary 32 of the transformer 33 and .the remainder of the system will not function.

When a pulse indicative of a difference between the voltage at a respective location 10a 1911 and the reference voltage 12 is generated in the primary winding 32 of the transformer 33, a corresponding pulse is induced in the secondary of the transformer 33. The secondary of the transformer 33 comprises two windings, an upper winding 36 and a lower winding 37, and a center tap 38 which is connected from the junction `of the two windings 36 and 37 to ground 19. As is well known, this configuration of the transformer secondary produces pulses of opposite polarity .at the upper end of the upper winding 36 and at the lower end of the lower winding 37 in response to a pulse in the primary winding 32.

Connected to the upper end of the upper Winding 36 and the lower end of the lower winding 37 are the cathodes of the respective diodes 40 and 41. As poled, the diodes 40 and 41 will pass only negative pulses and will not pass positive pulses. The negative pulses which are passed by the diodes 4Q and 41 are applied to one end terminal 43 of a potentiometer 45. The other terminal of potentiometer 45 is connected to ground 19. The error signal which results from the comparison of the monitored voltage with the reference voltage is developed across the potentiometer 45 which is used as a sensitivity control for the monitoring system.

To illustrate the operation of the Itransformer 33 and its associated diodes 46 and 41, two cases are considered. In the rst case, consider that a voltage is present at one of the locations lita 16m which dilfers from the reference voltage 12 in such a .manner so lthat a positive pulse is produced in the primary winding 32, that is, the upper end of the primary winding 32 assumes a positive polarity lwith respect to the lower end. This type of pulse, in turn, induces a positive polarity pulse in the secondary of the transformer 33 between the upper end of the upper winding 36, and ground 19. A negative pulse is produced between the lower end of the lower Winding 37 and ground 19. As previously explained the diodes `40 .and 41 are so poled that they pass only negative pulses. ln this instance, the negative pulse present at the lower end of the lower winding 37 is -passed by the diode 41 to the terminal 43 of the potentiometer 45.

In the second case, consider a voltage present at one of the locations lita ltn which ydiffers from the reference voltage 12 in such a manner so that a negative pulse is produced in the primary winding 32, that is, the upper end of the primary winding 32 assumes a negative polarity with respect to the lower end. This negative pulse in turn causes a negative pulse to -be present between tne upper end of the upper winding 36 and ground 19. This negative pulse is passed through the diode 40 to the terminal 43 of the potentiometer 4S.

As shown in the drawing, the circuit is arranged to `change pulses of any polarity, which are produced in the primary winding 32 of the transformer 33, into an error signal of negative polarity at -the terminal 43 of the potentiometer 45. As an alternative arrangement, an error signal may be produced at the terminal 43 in response to only a positive or a negative pulse in the primary winding 32 of the transformer 33. In the former case, the diode 4@ is not connected to the terminal 43 and the upper winding 36 may be eliminated, if desired. In the latter case, the diode 41 is not connected to the terminal 43 and the lower winding 37 may be eliminated, if desired.

Separate indications of a high or low voltage condition may be had by connecting the diodes 40 and 41 to dilferent sensitivity controls and amplilier systems.

A portion of the voltage which is impressed across pctentiometer 45 between the terminal 43 and ground L19 is supplied to a sense amplifier 47 by means of the variable arm 4d of the potentiometer 45. The function of sense ampiier 47 is to produce two distinct output signals which are indicative of whether the error signal formed as a result Iof the voltage comparison at junction point 21 is of a high order or `low order magnitude, that is, .a `large or a small difference between the voltage monitored from a location ta 1611 and the reference voltage 12.

As shown, the sense amplifier 47 is a three stage, direct coupled amplifier consisting of the three transistor empl-itiers Sil, 52 and 54. The transistor amplilier Stil is illustratively and schematically shown es being `of the PNP t pe and having an emitter electrode 57, a base electrode S3, and a collector electrode S9. Transistor amplitier 52 is illustratively and schematically shown as being of the NPN type and having an emitter electrode `61, a base electrode 62 and a collector electrode 63. The transistor ampliner 54 is of the same PNP type as the transisc tor amplifier S9' and has an emitter electrode 66, a base electrode 57 and a collector electrode 68. While the invention is described as using PNP type transistors for the amplifiers 5t? and 54 and an NPN type transistor for the amplifier 52, it should be realized that any suitable type transistor may be used for the respective amplifiers upon the application `of proper polarity signal and biasing voltages to it.

rFransistor amplifier 5@ is connected as a grounded emitter amplifier. The emitter electrode 57 is connected to ground 19 and a reverse bias voltage is applied to the collector electrode 59 from a suitable source of negative potential 7d by means of a resistor 7?.. A positive bias voltage is applied to the base electrode 53 from a suitable source `of positive potential 74 by means of a resistor 75. When the pulse of negative polarity error signal is yapplied from the center arm 46 of the potentiometer 45' to the base elect-rode 53 the positive bias voltage on the base electrode 58 is overcome and hole current iiow is established between the base electrode 53 and the collector eiectrode 59. rPhe negative pulse error signal applied to the base electrode 58 from the potentiometer `45l center arm 46 is inverted and appears at the collector electrode S' as a positive pulse. This positive pulse also appears at the base electrode 62 of the transistor amplier 52 which is directly coupled to the collector electrode 59 of the transistor amplifier Sti. The transistor amplifier 53 is connected as a common emitter amplifier. The emitter electrode e1 is supplied with a forward bias voltage from the negative potential source 76 by means of a resistor 76. A reverse bias is established on the collector electrode 63 from the positive potential source 74 by means of a resistor '77. The resistor 76 in the emitter circuit of the transistor 52 also functions to provide degeneration for temperature and gain stability.

The positive pulse signal applied to the base electrode 62 of the transistor 52 is inverted at the collector electrode 63 and is applied to the base electrode o7 of the transistor 54. The transistor 5'4 is connected as a grounded emitter amplifier and has its emitter electrode 66 connected to ground 19. A reverse bias is supplied to the collector electrode 68 from the negative potential source 70, by means of a resistor 78. The transistor 54 is biased beyond cutoff by means of the positive potential supplied from the positive potential source '74 to the base electrode 66 through the resistor 77. When the negative going output pulse of the transistor 52 is of a small magnitude, it is linearly amplified by the transistor 54 land appears across resistor '73 as a positive going output pulse on output line St? which is connected to the collector electrode 63. The output signal on `line 8@ is called the fine error signal since it is indicative of only a small error in the comparison of the two voltages at junction point 21. When the negative output signal at the collector electrode 63 of the transistor amplifier 52 is of a large magnitude, transistor S4 saturates and the collector electrode 63 of the transistor 52 is effectively grounded through the current path formed by the base electrode 67 and the emitter `electrode v65 of the transistor S4. In this case, transistor 52 functions as ,an emitter follower driver and a signal is produced on line S2. The signal on 'line 32 is called the coarse error signal since it is indicative of a large discrepancy in the comparison of the two voltages at junction point 21.

Therefore it is seen that an error signal of small magnitude produced at the terminal 43 causes the sense amamplifier 47 to function as la direct coupled amplier with complementary symmetry and degeneration for temperature and gain stability. When a signal :of ia large magnitude appears at the terminal 43, transistor Sti continues to function as an amplifier, but since the transistor 54 saturates, transistor S2 now functions as an emitter follower. By ladjusting the center arm 46 on the potentiometer 45 the sensitivity `of the sense amplifier -47 may be adjusted for individual voltage tolerances. In this manner, only differences between the voltage levels present at the various locations 16a 'itin and the reference lvoltage l2, which `are above a certain magnitude will cause the sense amplilier 47 to operate. The bias Voltage applied to the base electrode 67 of the transistor 54 may be varied in order to determine the magnitude of error signal which is necessary to cause transistor 54 to saturate and therefore cause the production of the coarse error signal online S2. This arrangement also provides a threhold to keep small error signals and noise from driving the transistor Sd to saturation.

Connected to the coarse error signal output line `trZ is an interlock circuit 83. ln a preferred form of the invention, it is desired :to cut off ythe power which supplies the voltage to 'the various locations 10a 1011 when a 'coarse error signal, which is indicative of a large discrepancy between the voltage lat a location and the reference voltage, is produced. This is done .to prevent possible `damage rto the system bythe abnormally high voltage. The block 83 may contain any suitable device which is capable of preventing the application of Jthe voltages to the locations lila 1G11. Suitable devices may be a relay, la voltage sensitive circuit breaker, etc., which are adjusted to operate only when ythe signal on line 82 is of a magnitude sulicient to be classied as a coarse error. This occurs when transistor 52 is operated as an emitter-follower.

Tlhe sequential monitoring of the various locations n permits time sharing of the sense amplifier CII 47 and other circuitry. Since sequential monitoring is used, it is desirable to provide an indication which shows wlhich location had the voltage which caused the production of the error voltage. 'Ilhis indication should be maintainable for a period of time which is at the control of the personnel who are going :to clear ythe trouble which :originally caused the error voltage to ybe produced. To accomplish this, the fine error output signal on line is sequentially connected to an indicator circuit comprising a series of indicator-s 84a, 84h, S40 8411, which correspond yto the locations 10a, 10b, 10c 1011. The indicators 84a 8411 are preferably miniature thyraftrous. As is well known, tlhe lthyratron is la gas-filled tube which gives off a visible glow when the gas inside the tube is ionized. The ionization of @the gas molecules occurs when the thyratron is red and is conducting.

As shown, each of the thyratrons 34a 84u has a respective plate electrode 85a v8511 and ya respective grid electrode 9de 9h11, which control-s the firing of its respective thyratron. The plates 85a 8511 of the uihyraitrotn tubes 84a 8411 are connected :to a source of positive potential represented by the battery 85 through a normally closed switch S7 and the respective resistors 38a, 8815, 88C 3811. The positive potential source S6 is preferably separate from mbe positive potential source 74 so that the `tl'iyratnons will not ybe affected when the power to the system is `disconnected by the operation of the interlock 83.

The ring of a mhyratron tube occurs when the negative potential supplied from a yseparate negative potential source 91 to the grids 90a, 90b, 9de 90u by means fof the respective resistors 98a, 98]), 98e 9311 is overcome =by che line error signal present on line 80.

The tine error voltage is sequentially applied to the grids 9G12, 99h, 99C 9tli1 'by means of fthe stationary Contact points 92a, 92h, `92e 9211, which are connected rto the line 80, and the movable contacts 94a, 94h, 94e 9411 which are connected to che grids of the Ithyra- 4tron-s fthrough diodes 96a, 96h, 951C 9611. '.[lhe movable contacts '94a 9411 are preferably connected to the distributor device which operates the movable contacts 14a 4M11 in lsuch` a manner so that the contacts 92 and 94 close before the contacts 14 and 2S. "Ilhis insures @hat the error Voltage :will be applied to the ahyratron grids yduring the full period 'of its production. If desired, the movable contacts 94a 9411 may be operated by `a separate distributor device, if synchronized with 14a 14n.

The ydiodes 96a 9611 are vbiased by connecting their respective cathodes to the negative potential source 91 through respective resistors 85: 98:1. A line error signal on line 80 is of a suflicieut positive polarity to overcome the negative bias 'on a ydiode 96 and be conducted 'to a grid 99 yof a tthynatron 84, thereby causing it to conduct. When one of the stationary contact points 92a 92u is contacted by a corresponding movable contact 94a 94u, and a une error signal is present on line 8@ during fthe period that 'the contact is made, 'the respective `diode 96 which is connected to the line Si) during the period of Contact will conduct land ycause a signal to lbe placed on the grid of the respective thyr-atron 84 to which it is connected. This signal causes fthe thyraftron 84 to fire and present a Avisible indication Ito an observer. The particular thyratron 84a 8411 which is fired corresponds rto a particular location 10a ln.

To illustrate the Ioperation of Ithe indicator circuit, assume for example that a voltage is monitored at location 10c which is of a :different magnitude than the reference voltage 12. As previously explained, this gencrates an error signal at the center arm 46 of the potentwmeter 45 which in turn, if the magnitude of the error signal is mot too great, causes a signal to be produced only on fthe fine error ioutput line 80. As location 10c is monitored by movable Contact 14C, movable contact of the indicator circuit, is brought into contact with r, i tithe iine error output line 89. The signal present on line `Si? is conducted through diode 96C to the grid 9de of the thyratron S40. T-his signal ycauses the thyratron Sile tto lire and give oli a visible gaseous glow. ln this mauner, an indication is provided of the location llc which had the voltage which produced the error signal. Errors present in any of the other locations -itia itin would the manifested in the same manner as the indicator circuit. It will be understood that a large discrepancy in the comparison of the .two voltages at junction point 21 will also elect openation of ythe indicator circuit since the `developnnent of a coarse error signal is always preceded by a fine error signal output on line Sil.

Switch 87 is used to clear the thyratron, which was fired, after the trouble at the location which caused the production of the error voltage has `been repaired. When switch 87 is depressed, the positive potential from the source de is removed from the plates tizia zc of all of the -thyratrons 3dr: ic. Once the positive potential from the source Se is removed, the conducting thyratron ceases to lire and the gaseous indication is no longer present.

A test switch lili is provided in order to check the indicator circuit. When switch lill is connected to ground i9 a voltage divider is formed by resistor Z2 and a resistor w3, lwhich is connected between junction point 2i and ground i9. The closing of the test switch lill. changes the magnitude of the reference voltage 12 at the junction point 21 and therefore when the Icorrect voltage is monitored at the locations lila ft an error signal is produced at the potentiometer center arm 46. This error signal causes the thyratrons 35a n to sequentially fire in `a manner `which has been previously explained, and all of the thyratrons remain conducting until the clearing7 switch 87 is depressed. When the switch itil is removed from the ground i9 position, the monitoring system is again ready to operate to monitor the location lila 1011.

Another indicator thyratron 196 is also connected to the fine error line di?. Thyratron 1% lires and give-s a visible indication, any time a fine error signal appears on line Sh. The indicator lilo is necessary when a plurality of voltage points at each of a plurality of locations are being monitored `by a plurality of sense amplifiers similar to the sense amplifier 47. For example, if five separate voltage points Vare to be monitored at each `of the respective locations lila 15in, there would be a movable contact, similar to contact 14, and stationary contact point, similar to contact point 2, for each of the voltage points at each location. The voltage present at corresponding points of the various locations should be of equal magnitude so that a single sense ampliiier may be used for all of the corresponding points', However, it should be realized that the voltages present at each of the points at a particular location need not be equal since each of the sense ampliiier-s may be adjusted to compare `a Voltage of a diffe-rent magnitude or polarity. A separate pulse transformer, and sense ampliiier, iive in this example, would be connected to each of the five contact points at a single location, for example, location 16a. The iive pulse transformers and sense amplifiers would then be connected to the ive voltage points at location lilb, and so on. In this mmner, each of the voltage points at each location is monitored. Each of the plurality of voltage points at a respective location may be monitored simultaneously, or sequentially, as preferred.

The fine error output lines of each of the live sense ampliiers used is simultaneously connected to the grid 90a of indicator 85a, by means of the diodes 96a, lilla etc., when the voltage points at location lha are being monitored. As many diodes are connected to the grid 96a of :the thyratron 84a as there are sense amplifiers. After the voltages at the five points at location lila are monitored, the live transformers and sense amplifiers are switched from the live voltage points at location lila to the corresponding five vol-tage points at location 1Gb. Just prior to this, the five iine error output lines are simultaneously connected to the thyratron 84h by means of the diodes 9%, llilb, etc. This procedure is followed until all of the voltage points at all o the location-s have been monitored.

Each or" the sense amplifiers 47 also has m indicator tube, similar `to thyratron lilo which is continuously connected to its fine error output line. When an error signal is produced by one of the voltage points at one of the locations, the particular voltage point is indicated by the firing of the indicator tube which is connected to the sense amplifier which lis associated with the particular voltage point. The particular location of the error producing voltage is indicated by the firing oi one of the thyratronsSSa 15511. Utilizing this anrangernent, the monitoring oi an electronic system having thirty locations with five voltage points for each location, for a total of iSd voltage points, would require only ltlnrty indicators 84, one for each of the thirty locations, and ve indicators M6, one for each of the iive voltage points at a location. in this manner, thirty-five indicators provide information for sepan-ate points.

it is obvious to one skilled in the art that the system described may be extended for used with any number of locations with any number of voltage points in respective locations.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in :the form and details of the device illustrated and in its opera-tion may be made by those skilled in the art, without departing from the spirit of the invention. it is the intention, therefore, to be limited only as indica-ted by the scope of the following claims.

What is claimed is:

l. A system for monitoring the voltages present at a plurality of locations comprising means for sampling the voltages at 'the respective locations, a source of reference potential, means for producing an error voltage when the voltage sampled is not substantially equal to said reference potential, sense amplifier means connected to said last named means for measuring the magnitude of the error voltage, said sense amplilier comprising first, second and third transistor ampliiiers, each of said transistor amplifiers yhaving a respective emitter, base and collector electrode, means connecting the collector electrode of said iirst transistor to the base electrode of said second transistor, means connecting the collector electrode of said second transistor to -the base electrode of said third transistor, means `for supplying the error voltage to the base electrode of said iirst transistor, iirst output means connected to the collector electrode o-f said third transistor, said iirst, second and third -tnansistors operating as substantially linear ampliers in response to an error voltage of less than a predetermined magnitude and thereby producing an youtput signal at said first output means, and second output means connected to the emitter electrode of said second transistor, said third transistor lbeing saturated by `an error voltage of a magnitude greater than said predetermined magnitude and causing an output signal to lbe produced at said second output means, a plurality of indicators, each one of said indicators corresponding to a respective one of said locations, and means for connecting the indicator corresponding to the location whose voltage is being sampled to said first output means of said amplifier during the time the voltage is sampled, a respective indicator producing an indication which represents the location where the sampled voltage did not substantially equal the reference potential in response to an output signal at said iirst output mem-s.

2. A monitoring system as set forth in claim l wherein means kare connected to said 'second output means for removing the IVoltage yfrom said locations in response to a signal at I.said second output.

References Cited in the file of this patent UNITED STATES PATENTS 5 Willis Sept. 20, 1938 Wilson Sept. 10, 1946 Weiner Mar. 18, 1952 Bixby July 14, 1953 Mlarkow et al. Dec. 7, 1954 10 10 Grillo May 8, 1956 Stanley Apr. 16, 1957 McVey Sept. 23, 1958 Keonjian Mar. 3, 1959 Donald Mar. 10, L1959 Fthenakis Oct. 20, 1959 Kidd et al. Sept. 29, 1959 Belland June 28, `1960 Rabier Nov. i1, 1960 Seeley May 2, 1961 

1. A SYSTEM FOR MONITORING THE VOLTAGES PRESENT AT A PLURALITY OF LOCATIONS COMPRISING MEANS FOR SAMPLING THE VOLTAGES AT THE RESPECTIVE LOCATIONS, A SOURCE OF REFERENCE POTENTIAL MEANS FOR PRODUCING AN ERROR VOLTAGE WHEN THE VOLTAGE SAMPLED IS NOT SUBSTANTIALLY EQUAL TO SAID REFERENCE POTENTIAL, SENSE AMPLIFIER MEANS CONNECTED TO SAID LAST NAMED MEANS FOR MEASURING THE MAGNITUDE OF THE ERROR VOLTAGE, SAID SENSE AMPLIFIER COMPRISING FIRST, SECOND AND THIRD TRANSISTOR AMPLIFIERS, EACH OF SAID TRANSISTOR AMPLIFIERS HAVING A RESPECTIVE EMITTER, BASE AND COLLECTOR ELECTRODE, MEANS CONNECTING THE COLLECTOR ELECTRODE OF SAID FIRST TRANSISTOR TO THE BASE ELECTRODE OF SAID SECOND TRANSISTOR, MEANS CONNECTING THE COLLECTOR ELECTRODE OF SAID SECOND TRANISTOR TO THE BASE ELECTRODE OF SAID THIRD TRANSISTOR, MEANS FOR SUPPLYING THE ERROR VOLTAGE TO THE BASE ELECTRODE OF SAID FIRST TRANSISTOR, FIRST OUTPUT MEANS CONNECTED TO THE COLLECTOR ELECTRODE OF SAID THIRD TRANSSISTOR, SAID FIRST, SECOND AND THIRD TRANSISTORS OPERATING AS SUBSTANTIALLY LINEAR AMPLIFIERS IN RESPONSE TO AN ERROR VOLTAGE OF LESS THAN A PREDETERMINED MAGNITUDE AND THEREBY PRODUCING AN OUTPUT SIGNAL AT SAID FIRST OUTPUT MEANS, AND SECOND OUTPUT MEANS CONNECTED TO THE EMMITTER ELECTRODE OF SAID SECOND TRANSISTOR, SAID THIRD TRANSISTOR BEING SATURATED BY AN ERROR VOLTAGE OF A MAGNITUDE GREATER THAN SAID PREDETERMINED MAGNITUDE AND CAUSING AN OUTPUT SIGNAL TO BE PRODUCED AT SAID SECOND OUTPUT MEANS, A PLURALITY OF INDICATORS, EACH ONE OF AID INDICATORS CORRESPONDING TO A RESPECTIVE ONE OF SAID LOCATIONS, AND MEANS FOR CONNECTING THE INDICATOR CORRESPONDING TO THE LOCATION WHOSE VOLTAGE IS BEING SAMPLED TO SAID FIRST OUTPUT MEANS OF SAID AMPLIFIER DURING THE TIME THE VOLTAGE IS SAMPLED, A RESPECTIVE INDICATOR PRODUCING AN INDICATION WHICH REPRESENTS THE LOCATION WHERE THE SAMPLED VOLTAGE DID NOT SUBSTANTIALLY EQUAL THE REFERENCE POTENTIAL IN RESPONSE TO AN OUTPUT SIGNAL AT SAID FIRST OUTPUT MEANS. 